Hydrocarbon polymers generally fall into two broad classes, thermoplastic and thermosetting resins. Thermoplastic resins may be readily worked by heating the polymer up to at least its softening point or melting point. They may then be processed by such conventional deformation methods as vacuum forming, extrusion of a melt, compression molding, etc.
The thermoset resins can generally not be reworked once they have hardened. In general, thermoset resins owe their unique properties to covalent cross-links between polymer molecules. The cross-links may be introduced by interaction of various monomers such as copolymerization of styrene in the presence of smaller amounts of divinyl benzene or the reaction of epoxy type resins with polyamines.
Uncured elastomers such as natural rubber and butyl rubber are thermoplastic. They may, however, be cross-linked or vulcanized by the use of sulfur and accelerators which react with the carbon of the unsaturated bonds in the polymer molecules to form, in effect, a thermoset product which can no longer be fabricated or worked except by machining or similar techniques. The characteristic property of a cross-linked polymer is its intractability above the softening point or melting point normally observed in the uncross-linked base polymer. Thus, whereas the uncross-linked polymer has a marked softening point or melting point above which the polymer is fluid and deformable, the cross-linked polymer retains its shape and will tend to return to that shape when deformed at all temperatures at which the polymer is stable. Once cross-linked the polymer is no longer fabricable, except possibly by machining. Cross-linked polymers have found wide utility because of the significant improvement in the physical properties obtained by cross-linking. Thus, rubber elasticity, impact resistance, flexibility, thermal stability and many other properties are either introduced or improved. The cross-linking of non-elastomeric polymers increases the toughness and abrasion resistance of the material. The intractability of thermosetting polymers limits the modes of processing the material into desired shapes.
A third class of polymers recently developed are ionically cross-linked polymers, generally referred to herein as "ionomers" or "ionomeric polymers". The cross-linking is accomplished by ionic bonding by way of association of ionic groups which are part of the polymer structure rather than by covalent bonding as is normal with thermoset polymers. The ionic aggregates therefore act as physical cross-links. These polymers exhibit the improved and desired properties normally related to thermoset polymers at normal operating temperatures while at elevated temperatures. The ionic aggregates dissociate allowing the ionomer to become deformable in the same manner as thermoplastic polymers. Unfortunately, in a majority of instances the ionic aggregates dissociate at temperatures close to or higher than the temperature at which decomposition occur in the ionomeric polymer composition and have, therefore, had limited usefulness.
The ionomeric polymers or ionomers may be readily prepared by a variety of techniques using numerous homo-, co-, and terpolymers as backbones. However, while all ionomers have several obvious advantages, one disadvantage to all is the increased difficulty in processability as compared to similar polymers having the same backbone but without ionomeric cross-linkages. It has been suggested that the processability can be improved by modification of the ionomer compositon. For example, U.S. Pat. No. 3,847,854 and U.S. Pat. No. 4,131,587 suggest that ionomers can be made readily workable by the addition of materials which preferentially plasticize the ionic cross-like regions to cause relaxation of the ionic bonds. Although these prior art mixtures are more readily processable, the final product either requires removal of the plasticizer to regain its desired properties or will have modified properties. Extraction is not always feasible nor complete and thus the final product does not exhibit the same properties as the neat starting ionomer.
U.S. Pat. No. 3,624,189 also discusses a mode of modifying polymer compositions to enhance its processability by the addition of a polar group containing modifying agent to the polymer composition. Such a blend presents a new composition which will exhibit characteristics attributable to the combination of the modifier and the polymer.
It is desired to provide a method of processing neat ionomers or ionomer containing compositions into shaped articles without modification of the polymer or the composition. Such articles would have to be capable of exhibiting the enhanced properties normally attributable to cross-linked ionomers.